Step 3 is to set out the values of
kan, kwn, kcn and mn
for each metal layer, and use these to calculate the value of L.

metal layer

1

2

3

4

5

6

kan

100%

100%

100%

100%

200%

0%

power metal allocated coefficient

kwn

80%

80%

80%

80%

80%

0%

power metal used coefficient

kcn

100%

100%

100%

100%

100%

0%¹

conductivity coefficient

mn

30%

30%

30%

30%

30%

0%

core area blocked

¹no M6 layer

The value of L depends on p which we don't know.
We iterate to the solution and use p=0 for the first estimate.

L =

kw1kc1(1-ps)(1-m1(1-ka2p)(1-ka3p))+

kw2kc2(1-m2(1-ka2p)(1-ka3p))+

kw3kc3(1-m3(1-ka2p)(1-ka3p))+

kw4kc4(1-m4(1-ka2p)(1-ka3p))+

kw5kc5(1-m5(1-ka2p)(1-ka3p))

=

( 0.44 + 0.56 + 0.56 + 0.56 + 1.12 )

=

3.24

Step 4: Calculate the power strap allocation percentage p.
The solution must be iterated, and the calculation below shows the first iteration.

m1′ =

m1×(1-ka2p)(1-ka3p)

p =

{

Vddmin×Pnom

−kc1×ps(1-m1′)

}

×

1

(Vcore−Vmin)×Vdd2×G

L

=

{

1.164×1

−1×0.22×(1-0.3)

}

×

1

(1.155−1.08)×1.22×25

3.24

=

(0.430−0.156)×0.309 = 8.49%

As shown on the right, a spreadsheet can be used to iterate to the answer
of p=7.70%.

Step 5: Calculate the new core size.
If the initial core size estimate without power straps is x,
then with power straps the core size becomes x′

x′ =

x

=

x

=

x

= x+8.34%

√(((1-ka2p)(1-ka3p))

√0.92302

0.9230

The value 8.34% is called the IR Drop Adder.

The width of the power straps is set by the pitch,
strap allocation or width chosen by the user and the value of p
just calculated. An example is shown in the table below,
where we set the supply strap allocation to 5.5µm and compare
it to the old solution.

The new solution has allowed the vertical power strap pitch to go up from
one every 64µm to one every 143µm.
The core side is 1004µm less and the core area is 20% less
than the first solution, due to double bonding of supply pads,
tighter Vddmin spec and wider metal-5 straps.